The resistance spot welding process is commonly used to make spot welds to join sheet metal parts. For example, thousands of spot welds are used in assembling the automobile. Therefore, improving the resistance spot welding characteristics has direct impact on the cost as well as the quality of automotive productions. Electrical contact resistance at electrode-to-sheet metal and sheet-to-sheet metal interfaces is an important characteristic to determine in order to optimize the welding process. There are currently two methods used to obtain electrical contact resistance measurements. One method is called an on-line approach, and the other one is called an off-line approach. The on-line approach indirectly monitors the variation of contact resistance during the real spot welding process. A problem with the on-line approach is that because there is an increase in heat locally, it is difficult to obtain the contact resistance property at a controlled temperature.
The off-line approach measures the contact resistance by simulating conditions during the real process, such as temperature and compressive load. A small current, as typically used in multimeter measurements, is used to measure the resistance in the off-line approach. A problem with the off-line approach is that, because of the existence of surface oxide or contaminant film, the measurements are strongly dependent upon the surface conditions and thus measurements are often inaccurate.